The present invention relates to a method for attenuation compensation in a communication system, and more exactly to a method to adapt the band-width as a result of signal attenuation.
To generate a high EIRP in a ground/space system either a large antenna or a transmitter having high output power is needed. A large antenna will become bulky, besides it will produce a smaller coverage while producing the higher output power and additionally it will become more expensive. In certain cases a part embracing more than 10 dB of the link budget is reserved only for compensation of weather conditions, which is one major reason for losses in microwave communication systems of this type. However, such conditions are prevailing only during a very small percentage of the time.
A further problem is that many satellite systems are interference limited, which implies that the entire dynamic range for weather changes and eventual changes in distance between transmitter and receiver must be compensated by a regulation of the transmitter output power. Dynamically this leads to that the transmitter working point for certain cases must be changed by more than 20 dB.
There are primarily found a number of Japanese documents addressing this problem. A document JP 63-262924A discloses a device for satellite communication, in which a decision circuitry calculates the excess of a precipitation attenuation over the range of power control and then changes the data rate to satisfy the prescribed line quality. Another document, JP 41-3044317A, also contemplates a way to keep the actual quality of a satellite communication line constant at all times by measuring a BER (bit error rate) of a loop-backed pilot signal and adjust transmission speed of the pilot signal to maintain a preset bit error rate independent of the channel attenuation. The result of the pilot signal measurement is then controlling the transmission power.
Still another document JP 21-13653A describes a method to effectively utilize frequencies in a simple constitution, and to effectively make them to function for various types of fading by providing a large capacity transmitting system and a small capacity transmitting system. The receiving condition is supervised by means of a receiving system, and a switching of transmitting systems between the transmitting side and receiving side takes place assisted by a switching control signal being the supervisory result. The small capacity transmitting system utilizes a narrowed transmission bandwidth. However the regulation takes place only in two steps and requires the equipment to be doubled.
Yet another method of addressing the problem is demonstrated by a document FR 2,757,725, A. This method utilizes several carriers in parallel, each of which periodically generates a symbol. The symbol occupies a bandwidth, which is divided into two sub-bandwidths and provides two half symbols. Thus each such bandwidth can transfer a xe2x80x9chalf symbolxe2x80x9d. Depending on the transmission conditions, a half or complete symbol may be transmitted.
It has also for instance been found suggestions to involve more coding in situation of bad weather, which also decreases the resulting net data rate.
Therefore there is a desire to present a method, which can still produce a good total efficiency of a system even if the data rate for a specific channel is decreased due to attenuation and fading of the specific link due to for instance precipitation for that specific path. If it would be possible to avoid an over-dimensioned link budget, it would result in cheaper and smaller ground terminals and possibly also cheaper and smaller satellites and/or higher transfer capacities.
The present invention discloses a method in which the bandwidth is decreased instead of increasing EIRP in order maintain a same power density per Hz at the receiver in cases of poor weather conditions resulting for instance in precipitation attenuation or fading. This also leads to a reduced data transmission rate for the specific channel due to the poor weather conditions, but different from the state of the art, the reduced bandwidth instead allows more subscribers to be connected. Furthermore the dynamics of the transmitter output power regulation will only need to cover the steps of bandwidth regulation. If the bandwidth is reduced, for instance, by dividing by a factor two it will be enough with an order of 3 dB dynamics or xc2x11.5 dB in the regulation of the transmitter output power.